As a shape of a conventional electric double layer capacitor, there may be a coin type wherein an element having a separator sandwiched between a pair of polarized electrodes composed mainly of activated carbon formed on current collectors, is accommodated together with an electrolytic solution in a metal casing, which is then sealed by a metal cover via a gasket, or a cylindrical type wherein an element having a pair of polarized sheet electrodes wound with a separator interposed therebetween, is accommodated together with an electrolytic solution in a metal casing, which is then sealed so that the electrolytic solution will not evaporate from an opening of the casing.
Further, as one for a large current and large capacitance application, a lamination type electric double layer capacitor has also been proposed wherein an element having many polarized sheet electrodes laminated via a separator disposed therebetween, is incorporated (JP-A-4-154106, JP-A-3-203311, JP-A-4-286108). Namely, rectangular polarized sheet electrodes are used as positive electrodes and negative electrodes, and they are alternately laminated with a separator interposed therebetween, to form an element, which is then accommodated in a casing in such a state that positive electrode lead members and negative electrode lead members are connected by caulking to the terminals of the positive and negative electrodes, respectively, then the element is impregnated with an electrolytic solution, and the casing is closed with a cover.
As an electrolytic solution for a conventional electric double layer capacitor, not only an aqueous electrolytic solution containing a mineral acid such as sulfuric acid, an alkali metal salt or an alkali, but also various organic electrolytic solutions have been used. As the solvent for such organic electrolytic solutions, propylene carbonate, γ-butyrolactone, acetonitrile, dimethyl formamide (JP-A-49-068254) or a sulfolane derivative (JP-A-62-237715), has been known. When the withstanding voltages are compared, the aqueous electrolytic solution has a withstanding voltage of 0.8 V, while the organic electrolytic solution has a withstanding voltage of from 2.5 to 3.3 V. The electrostatic energy of a capacitor corresponds to the square of the withstanding voltage. Accordingly, from the viewpoint of the electrostatic energy, the organic electrolytic solution is more advantageous. However, such a solvent having a high dielectric constant, in which the electrolyte is highly soluble, usually has a high viscosity. Therefore, although an electrolytic solution having a high electrolyte concentration will be obtained, no high electric conductivity will be obtained. Further, a chain ether, a chain ester, a chain carbonate and the like, which are low viscosity solvents, have a low dielectric constant, whereby substantially no electrolyte will be dissolved therein, and no high electric conductivity will be obtained.
Propylene carbonate which is commonly used as a solvent for an electrolytic solution of an electric double layer capacitor, is a solvent having a high dielectric constant and a relatively not so high viscosity, and is thereby preferably used. However, propylene carbonate may decompose when a voltage of about 2.9 V or higher is applied. Further, electrochemically stable sulfolane has a high withstanding voltage and will not decompose by application of a voltage of up to about 3.4 V, but the electrolyte will hardly soluble in it, and it has a high melting point, and it is particularly poor in low temperature characteristics.
On the other hand, as the electrolyte, a salt comprising as a cation (C2H5)4N+, (C2H5)4P+ or the like having a high electric conductivity has been used. Further, (C2H5)3(CH3)N+ or the like which is highly soluble in a solvent has been proposed. However, no sufficient solubility in a solvent having a low dielectric constant and a low viscosity has been achieved. Among solvents having a low dielectric constant and a low viscosity, a chain carbonate is electrochemically stable, but in dimethyl carbonate, ethyl methyl carbonate or the like as a single solvent, substantially no electrolyte can be dissolved.